A different kind of solar power

[Just some guy]

Well this is fairly simple so I guess there's a flaw in my 'plan' somewhere :p, but hey here goes.

Current solar cells are only about 25% efficient, however instead of using sunlight to directly generate electrical current why not use it as a heat source?

Of course sunlight is normally low grade heat, but why can't giant lens systems be used to focus the sunlight on a smaller area? Lenses are just ground class and they could be pretty crude, and the only energy losses would be from sunlight reflecting off the surface of whatever they're heating. If you heated a fairly volatile liquid with a low reflectivity (um...water with black dye? :p ok maybe something more refined than that) then you would only need to heat up the water to just a bit over boiling point to have a theoretical maximum efficiency of >25% (if your cold reservoir was the sea or something cold like that), and considering how tightly you could focus the sunlight the efficiency could be bumped to well over that of a conventional solar cell, and all that using ground glass and a simple heat engine.

So where's the flaw? why hasn't it been done? would the yeilds be too low to bother with the whole thing? Is making giant lenses harder than I think?

Cheers,
Zac.

 

[trajan22]

Im not sure about the efficiency of these but it sounds very similar to what you are describing.
http://www.renewableenergyaccess.com/rea/news/story?id=44696

 

[russ_watters]

You can, of course, also use solar to heat water for heat and hot water, with very high efficiencies. And you don't need black water - just black pipes.

 

[lpfr]

So where's the flaw? why hasn't it been done? would the yeilds be too low to bother with the whole thing? Is making giant lenses harder than I think?

Direct solar heating is currently used in Europe, mostly in Germany. Of course you can't use only solar heating, you need to complete with another kind of heating.

Lenses are big and must be oriented. What you do is use parabolic reflectors in the back of each pipe. Pipe panels are fixed and inclined to be perpendicular to the Sun.

Don't forget to put antifreeze in the water (some have).

 

[FrogPad]

Sunlight can be concentrated and used with a Stirling engine.
check:
http://en.wikipedia.org/wiki/Stirling_engine

There is also something called, I think thermionicsolar. It uses a semiconductor with a low band gap and gets energy from a heat source.

There are some problems with concentrators though mainly due to tracking. Also, with the use of multijunction cells, some people (I think spectrolab and boeing if I remember correctly) have achieved 39% efficiency.

Check out Fresnel lenses for a cheaper method to concentrating light.